1. Field of the Invention
The present invention relates to an electromagnetic interference (EMI) filter, particularly to an integrated circuit (IC) EMI filter with electrostatic discharge (ESD) protection incorporating inductor-capacitor (LC) resonance tanks for rejection enhancement.
2. Description of the Related Art
Low-pass filter circuit related to this invention is used to block incoming electromagnetic interferers to wireless communication electronic systems, such as a cellular phone. Such a low-pass filter circuit has several critical specifications (specs) to meet the system requirements, including a low pass-band insertion loss (IL), a broad pass-band and high rejection-band attenuation. A low insertion loss for the filter ensures the desired baseband signals passing through with as little energy loss as possible. A broad low pass-band, defined as the frequency bandwidth from direct-current (DC) to a cut-off frequency (i.e., fc) measured at the 3 dB insertion loss point, allows the desired baseband signals with wider frequency spectrum (i.e., lots of useful baseband harmonic signals) to pass through filter. Typically, a wider pass-band (i.e., higher fc enables higher wireless communication data rates. The rejection-band is determined by the wireless system applications, typically featured from 800 MHz to 6 GHz. The rejection band serves to remove any high-frequency Electromagnetic Disturbance (EMI) interferers, or, noises, which are generally associated with the carrier band frequencies in radio-frequency (RF) systems. To ensure the desired data rates and signal integrity, a −30 dB attenuation in the rejection-band for the EMI interferes is preferred in the EMI filter circuit designs, which means that the noise power must be reduced by a factor of 1000, to ensure the required signal-to-noise ratio (SNR) for the wireless systems. It is well known that a π-shape CLC type filter 10, shown in FIG. 1(a), can theoretically achieve the required low-pass filter function described above. Similarly, a π-shape capacitor-resistor-capacitor (CRC) type LPF circuit 12, as illustrated in FIG. 1(b), can be used to achieve the required filter function. FIG. 2 describes the typical filter insertion loss curve, or, called the forward amplification gain (S21) curve characterized in the S-parameter measurement in practical designs. However, in practical filter designs, to achieve the required low insertion loss and broad pass-band, while obtaining high rejection-band attenuation, are in conflict and very challenging, which requires careful filter circuit design trade-off and innovative design techniques. In particular, the S21 curve should have a very clean −3 dB cut-off frequency (fc) and a fast roll-off attenuation curve, i.e., a steep S21 curvature after the designed fc point. The conventional CLC filter circuit cannot achieve these requirements due to various integrated circuit (IC) and package parasitic effects. All prior arts may not satisfactory due to the circuit performance and the circuit complexity.
FIG. 1(a) shows the ideal CLC LPF filter circuit schematics, which is a classic third-order filter circuit. The filter circuit can be considered as a typical 2-port network consisting of the port 1 (input) and the port 2 (output) symmetrically. This basic CLC filter consists of two capacitors and one inductor to realize the low-pass filter function. FIG. 3 shows a practical CLC LPF filter circuit schematic including the unavoidable parasitic components and integrated ESD protection diodes. A resistance 14 is the series resistance associated with the conduction channel inductor 16, which causes the insertion loss due to resistive loss. Two inductance 18, one inductance 20 and one resistance 22 model the parasitic inductance and resistance associated with the bonding and package of the filter circuit, respectively. The capacitances 24 can utilize the junction capacitance of the integrated ESD protection diodes 26 (or other ESD protection devices). As shown in the filter schematics, any EMI interferers (i.e., noises) can be filtered out in each direction of the 2-port network. In a typical application scenario as illustrated in FIG. 4, the low-pass filter 28 is placed between the baseband IC chip 30 and the display 32 (e.g., a liquid crystal display, or LCD) port in a Smartphone printed circuit board (PCB). This filter allows the desired baseband signals pass through, while blocking the undesired high-frequency interferers emitted from the noisy LCD module. Some prior arts used fifth-order LC filter circuit and coupled inductors to enhance the filter performance. FIG. 5 depicts typical S21 measurement result for a conventional CLC EMI filter circuit corresponding to a conventional filter shown in FIG. 3. It supports a pass-band of about fc=320 MHz wide, good for high data rates up to 120 Mbps. However, the rejection-band attenuation at 800 MHz is only about −23 dB, which is less than the desired −30 dB target.
In view of the problems and shortcomings of the prior art, the present invention provides an integrated circuit (IC) electromagnetic interference (EMI) filter with electrostatic discharge (ESD) protection incorporating inductor-capacitor (LC) resonance tanks, so as to solve the afore-mentioned problems of the prior art.